Many to relationship in ef code first

Updating many to many relationships in Entity Framework Core – The Reformed Programmer

many to relationship in ef code first

Configure Many-to-Many Relationships in Code-First. Here Visit the Entity Relationship chapter to understand how EF manages one-to-one, one-to-many and. A many-to-many relationship is defined in code by the inclusion of collection properties in each of the entities: public class Book; {; public int. It's not possible to create a many-to-many relationship with a customized join table. In a many-to-many relationship EF manages the join table internally and.

Configuring Many To Many Relationships in Entity Framework Core

With the one-to-zero-or-one relationship, a Student can be saved without StudentAddress but the StudentAddress entity cannot be saved without the Student entity. EF will throw an exception if you try to save the StudentAddress entity without the Student entity. The HasOptional method configures the Address navigation property in Student entity as optional not required when saving the Student entity.

Then, the WithRequired method makes the Student navigation property of StudentAddress as required required when saving the StudentAddress entity; it will throw an exception when the StudentAddress entity is saved without the Student navigation property. Thus, you can configure a one-to-Zero-or-one relationship between two entities where the Student entity can be saved without attaching the StudentAddress object to it but the StudentAddress entity cannot be saved without attaching an object of the Student entity.

This makes one end required. EF API will create the following tables in the database. Configure a One-to-One relationship using Fluent API We can configure a one-to-One relationship between entities using Fluent API where both ends are required, meaning that the Student entity object must include the StudentAddress entity object and the StudentAddress entity must include the Student entity object in order to save it.

Configuring One To Many Relationships in Entity Framework Core | Learn Entity Framework Core

These will always be one-to-zero-or-one relationships. Address makes the Address property of StudentAddress as required and. Student makes the Student property of the StudentAddress entity as required. Thus it configures both ends as required. These foreign keys then form a composite primary key for the BookAuthor table. Its not hard, so let me give you the code to define the BookAuthor entity class. I have a property called Order because the order of the authors matter.

many to relationship in ef code first

EF Core will find the relationships using its by convention rules because I have used names that it understands: Because I used the classes Book and Author, which EF Core knows are part of the database, then it knows its a relationship. HasKey method towards the bottom of the code where I define the composite key consisting of the BookId and the AuthorId.

Configure Many-to-Many Relationships in Code-First

But if you want to explicitly define the relationships you can, using the. There are loads of configuration rules, which I cover in chapter 6 and 7 of my bookor you can look at the EF Core docs.

You can set the keys, but not the relationships, via attributes. Summary — how to add a many-to-many relationship To add a many-to-many relationship in EF Core you need to: Create a linking entity class that has the foreign keys s of the two entities you want to form a many-to-many link between. In my example these are BookId and AuthorId. It is efficient but not necessary to make these two foreign keys into the composite primary key.

This will ensure that there is only ever one link between the two entities you want. It is also more efficient in terms of size of the database table.

You most likely want a collection relationship to the linking table in one or both of the linked entities. The code below creates a new book with a new author. Add book ; context. SaveChanges ; Updating a many-to-many relationship It turns out there are two scenarios under which you want to update a relationship. You find this sort of update that happens in a console application, or inside your business logic see this link on business logic and EF Core Disconnected state: This is where the update is split into two halves: This happens on web sites, where in the first stage the user picks what they want done and posts it back.

The second stage then has to re-read the data and update it.

Configuring Many To Many Relationships in Entity Framework Core | Learn Entity Framework Core

I am going to describe these two approaches separately, so you can go to the one that fits the application you are building. Connected state update In the connected state we read in the Book entity and update it immediately, i.

many to relationship in ef code first

I am using a Sqlite, in-memory database, which I seed with four books with known titles and authors. Here I load the Book entity using an Include method to load the AuthorLinks at the same time, as tracked entities.

The default way of loading data in EF Core is as tracked entities, that is, EF Core takes a copy of the loaded data so that it can detect if any changes have happened since they were loaded.

You can turn off tracking by including the method. AsNoTracking to your query useful in read-only queries, as it makes them a little faster. Summary — Connected state many-to-many update To update a many-to-many relationship in the connected state: In my example I loaded the Book entity I wanted to change along with its AuthorLinks property, which holds the collection of the linking entities in the BookAuthor table. Alter the linking table collection, either by adding or removing entries from the collection.

Disconnected state The disconnected state happens when the initial read of the data and update of the data are done separately, i. This happens in a web application which has two stages: